Maize (Zea mays L.), is the most C4 crop in the world [1]. It comes in importance after the wheat and rice globally through production and area, due to its grown for both food and fodder [2], and its entry into many industrial purposes [3], its seeds are high in proteins, fats and vitamins (A, C, and E) therefore, it is considered an important strategic crop [4]. Maize grows in the most of Iraq's governorates, but it's yield is smaller than the other countries of the world [5]. Each environment has its own suitable varieties and hybrids, which differ among themselves not only in their genetic capacity but also in their response to modern agricultural methods and the recommended techniques. Studies have indicated a decrease in the productivity of any crop when the appropriate conditions for its growth are not available, including the available of the major and minor elements. Maize is one of the agricultural crops most affected by zinc deficiency [6]. Zinc deficiency is one of the Wide plant nutrition problems, it is one of the necessary micronutrients for plant by enhancing growth due to its effects as enzyme regulatory for photosynthesis and other physiological processes [7]. It is also important in the synthesis of the amino acid (Tryptophan) which is the basic material for the manufacture of Indole acetic acid (IAA), in addition its role in the metabolism of DNA and RNA nucleic acids. Sarwar et al. [8] when spraying maize with different concentrations of zinc (0, 0.2, 1.5, and 3 ppm), found that the concentration of 1.5 ppm achieved the best results of growth and productivity of maize. Liu et al. [9] indicated that adding zinc had a significant effect in increasing the chlorophyll content in the leaves and improving the efficiency of the photosynthesis process as well as increasing the grain yield of maize. Therefore, the study aims to find out the effect of spraying with different levels of zinc on some growth traits and the outcome of a group of maize cultivars and the overlap between them.

A field experiment was carried out during the agricultural seasons 2019 at AL- Qurna agricultural research station (80 km north of Basra center), to study the response of five varieties of maize (Fajer-1, Baghdad, Maha, Sarah and 5018) to foliar application of three Zn concentration. 0, 25  and 50 mg.L^-1, as zinc sulphate (ZnSo4.7H2O, 35% Zn) took the symbols Zn₀, Zn₁ and Zn₂. Experimental design was split plot according to Random Complete Block Design (R.C.B.D.) with three replicates. Zn concentrations was in the main plot considered, while varieties as sub-plots. Soil samples were taken from a depth of 0-30 cm and were analyzedaccording to methods described in Black [10] and Page et al. (Table 1). 

Table 1: Soil Physical and Chemical Properties

Planting date was in 10^th 0f August 2019. subplot Area was 20 m² (4*5m) including eight ridges about 5m length with 50 cm distance between them, 25 cm between plants and the density of plants was 15 plant /m². Nitrogen fertilization was applied with a quantity of 120 kg N/ha as urea (46% N) at three equal doses: sowing, six leafy and at tasseling stage [11]. Phosphate fertilizers applied as super phosphate (21% P) at the average of 60 kg P/ha at sowing (21% P). Zinc sprayed during the early morning at a rate of 400 L ha^-1 the first spray was 30 days after sowing. While, the second spray was 15 days later using handle sprayer (5 litter), a detergent solution was applied to the solution to decrease the surface tension of the spray solution

Data recorded

Five plants were randomly selected for each experimental unit to measure the following characteristics: plant height (cm), stem diameter (mm), leaf area (cm²), leaf area index. The crop growth rate (gm m^-2 day ^-1) was also measured according to the equation: CGR = (1 / A) × (W2–W1) / (T2– T1) [12]. 

The Net Assimilation Rate (gm m^-2 day ^-1) was also calculated according to the equation: NAR = ((W2– W1)/(T2 – T1))×((Log LA2– Log LA1)/(LA2– LA1)). [12]. 

Grain yield was determined and standardized to tons per hectare (t ha⁻¹) to ensure comparability across treatments.

Statistical analysis

Data were statistically analyzed using SPSS Program for split plot randomized complete block design (R.C.B.D) and means between treatments were compared by least significant difference test at 0.05 level of probability [13].

Physiological Growth Parameters (LAI, CGR, NAR)

The data presented in Table 3 indicate that zinc treatments significantly enhanced physiological growth parameters, specifically leaf area index (LAI), crop growth rate (CGR), and net assimilation rate (NAR). At a high zinc concentration of 50 mg/L, LAI increased to 2.69, CGR to 2.08 g m²/day, and NAR to 0.77 g m²/day. In contrast, the control treatment with no zinc (Zn₀) showed the lowest values, with LAI at 2.15, CGR at 1.62 g m²/day, and NAR at 0.75 g m²/day. Zinc is necessary for the synthesis of auxins, which are crucial for cell division and elongation, as well as for the formation of chlorophyll and photosynthesis, these findings correspond with Alshummary et al. [14]

Table 3 shows the significant effect of cultivars on physiological growth parameters, Variety 5018 recorded the highest values for LAI (2.59), CGR (2.24 g m-2 day-1), While the cultivar Maha recorded the highest value for NAR (0.82 gm m-2 day-1), the lowest values were recorded for cultivar Sarah for LAI (2.28), while Fajer-1 cultivar gave the lowest CGR (1.34 g m-2 day-1) and NAR (0.55 g m-2 day-1), The reason for the different response of cultivars could be attributed to the difference in genotype, this result agreed with Al-Freeh et al. [15].

Spraying with the Zn₂ level of cultivar 5018 gave a significant superiority for most of the studied traits, as it recorded the highest LAI (2.84) and CGR (2.51 g m-2 day-1), a combination treatment Zn₂ and cultivar Maha gave the highest NAR (0.88 gm-2 day-1), while the interaction between control treatment (Zn₀) and cultivar Sarah recorded the lowest values for LAI (1.96), CGR (1.68 gm-2 day-1), The combination of Zn₀ Fajer-1 and Zn₁ Fajer-1 gave the lowest NAR about 0.52 and 0.51 gm-2 day-1 respectively.

leaf Area

The data in Table 3 reveal that the Zn₂ treatment resulted in the highest leaf area at 4826.45 cm², whereas the control treatment (Zn₀) had a reduced leaf area of 3762.72 cm². The primary factor contributing to the increase in leaf area is the higher uptake of zinc, The main cause of the increase in growth parameter (leaf area) were a larger intake of zinc, which plays a role in metabolic activities within the plant and increases photosynthetic efficiency, it also participates in the production of the amino acid required for cell elongation [16-17].  The results refer that Variety 5018 recorded the highest values for leaf area (4575.27 cm²), while the lowest values were recorded for cultivar Sarah for leaf area (3989.37 Cm), this result agree with Alshummary et al. [14].  Spraying with the Zn₂ level of cultivar 5018 gave a significant superiority for most of the studied traits, as it recorded the highest leaf area (5095.10 cm ²), while the interaction between control treatment (Zn₀) and cultivar Sarah recorded the lowest values for the studied parameters, leaf area (3427.34 cm²)

Plant Height

Significant differences were observed among the zinc spraying treatments (Table 2). The treatment with 50 mg/L zinc resulted in the highest average plant height of 155.84 cm, whereas the control treatment had an average height of 120.64 cm. This may be due to the role of zinc in increasing the activity of many enzymes in the plant, which contribute to raising the level of growth processes and vegetative development of the plant, including the formation of hormones, proteins, and chlorophyll necessary for the formation of dry matter through vital processes within the plant, which is reflected in increased growth and then increase the height of the plant [18]. Or this may be attributed to the role of zinc in the synthesis of the amino acid tryptophan, which is considered the basic substance for making the plant hormone auxin, which is found in the growing apex of plants and is responsible for the height of the plant through its necessary requirements and its role in increasing cell division and elongation and the formation of the largest number of cells, which in turn leads to Increases plant growth and thus reflects positively on plant height [19].

Table 2. Effect of Varieties, Spraying With Zinc and Their Interaction on the Leaf Area, Plant Height and Stem Diameter 

Table 3: Effect of Varieties, Spraying With Zinc and Their Interaction on the Physiological Growth Parameters (CGR, NAR, LAI and Stem Diameter)

The results showed that there were significant differences between the maize cultivars in the plant height (Table 2), as the 5018-cultivar excelled by giving the highest average (153.44 cm), while the Baghdad cultivar gave the lowest average (129.56 cm). The difference between maize cultivars in the traits of plant height is due to the nature of the cultivar genetic makeup, which makes it more adaptable to the surrounding environmental conditions and thus is reflected in the exploitation of nutrients with high efficiency, which leads to increased accumulation of dry matter and thus increased plant height [20].

The results also showed that there was a significant interaction between the two study factors (Table 2), as the highest interaction was at average (172.93 cm) at concentration 50 mg L^-1 with the Baghdad cultivar, while the lowest interaction was at average (110.87 cm) in the control treatment with the cultivar Sarah.

Stem Diameter 

Different foliar concentrations of zinc had a significant impact on stem diameter. The Zn₂ treatment resulted in a larger stem diameter of 26.47 mm, while the Zn₀ treatment produced the smallest stem diameter of 22.16 mm. This effect is might due to zinc's role in enhancing physiological growth characteristics. The results of Table 2 indicate that the 5018 variety was superior, recording the largest stem diameter (25.91 mm), while the lowest values were recorded for cultivar Sarah for stem diameter (23.47 mm). Spraying cultivar 5018 with the Zn₂ level significantly improved most of the studied traits, achieving the highest stem diameter of 28.85 mm. In contrast, the interaction between the control treatment (Zn₀) and cultivar Sarah resulted in the lowest stem diameter of 20.99 mm.

Grain Yield

The results demonstrated that corn responds significantly to zinc spraying. Increasing the spray concentration from zero to 50 mg/L led to an 80.58% increase in grain yield compared to the control treatment, which produced 2.889 tons per hectare. In contrast, the Zn₂ treatment yielded 5.217 tons per hectare. This increase is likely due to zinc's role in enhancing the production of nutrients essential for seed growth in the plant, as noted by Fayadh and Al-Hadethi [21]. These findings are consistent with Kadhim and Jabar [22].  Results showed that cultivars had significant effect on seed yield, The highest seed yield was obtained from 5018 cultivar which reached 4.210 tons ha-1, did not differ significantly from the cultivars Baghdad and Maha (4.130- and 4.191-ton ha-1), while the lowest values were recorded for cultivar Sarah for yield (3.901ton ha-1). Table 2 shows that the interaction between zinc levels and cultivars has a statistically significant influence on yield. Spraying cultivar 5018 with Zn₂ led in the maximum production of 5.713 tons per hectare, while combining Zn₀ with the Maha cultivar produced the lowest yield of 2.420 tons per hectare.

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